In fire protection systems, automatic sprinklers are typically employed to discharge water onto a fire when a predetermined high-temperature condition is reached in the ambient environment. A conventional automatic sprinkler includes a sprinkler body having a fluid passage formed therein and a valve plug which is adapted to seal the fluid passage outlet. A temperature-responsive element is utilized to normally maintain the valve plug in sealing engagement with the outlet. When the predetermined high-temperature condition is reached, the temperature-responsive element ruptures, disassociates, or otherwise changes in shape to release the valve plug from sealing engagement with the fluid passage outlet and thereby actuate the sprinkler.
Conventionally, the temperature-responsive element may take a number of forms, including that of an alcohol-filled frangible bulb, a releasable soldered plate assembly, a fusible link, or a bimetallic element. In U.S. Pat. No. 3,561,537, for example, the temperature-responsive element takes the form of a bowed bimetallic element 80 (FIG. 5) that tends to straighten as the ambient temperature rises, thereby unlatching the lever 73 from behind the strut 71 and actuating the sprinkler.
It has been proposed, for example in U.S. Pat. No. 4,596,483 (incorporated by reference herein), to utilize a shape-memory alloy (SMA) as the temperature-responsive element in an automatic sprinkler. Specifically, as shown in FIG. 7 of the '483 patent, a linkage element 38 made from a shape-memory alloy includes a pair of coplanar jaws and is employed to normally restrain movement of the lever arm 35 away from the lever arm 36. In this condition, a cover 31 is maintained in sealing engagement with a threaded tube end 30, whereby water is prevented from being discharged from the sprinkler. When the temperature of the ambient environment rises due to a fire, the jaws spread apart from each other in a direction substantially perpendicular to the plane of the jaws (see FIG. 3). The linkage element 38 then pivots relative to both the lever arm 35 and the lever arm 36 (see FIG. 4) so as to release the lever arm 35 from the lever arm 36. The cover 31 is thus permitted to separate from the threaded tube end 30, thereby actuating the sprinkler.
In order to ensure safety in the operation of automatic sprinklers, organizations such as the Underwriter's Laboratories (UL) and the National Fire Prevention Association (NFPA) have promulgated standards and minimum specifications for both commercial and residential automatic sprinklers. For example, in order for a commercial automatic sprinkler to obtain a "Quick Response" rating under UL Standard "UL-199" (the entire text of which is incorporated by reference herein), the sprinkler having a 155.degree. F.-165.degree. F. rated element must actuate within about 14 seconds when mounted in a UL designed fixture and held in a 275.degree. F. air stream moving at a rate of about 8.33 feet per second. Other standards and minimum specifications for commercial and residential standards are set forth in UL Standard "UL-1626", "UL-199" and NFPA Standards "NFPA 13" and "NFPA 13R" (residential), all of which are incorporated by reference.